Spin-state splittings of iron(II) complexes with trispyrazolyl ligands

We report a computational study at the OPBE/TZP level on the chemical bonding and spin ground-states of mono-nuclear iron(II) complexes with trispyrazolylborate and trispyrazolylmethane ligands. We are in particular interested in how substitution patterns on the pyrazolyl-rings influence the spin-state splittings, and how they can be rationalized in terms of electronic and steric effects. One of the main observations of this study is the large similarity of the covalent metal–ligand interactions for both the borate and methane ligands. Furthermore, we find that the spin-state preference of an individual transition-metal (TM) complex does not always concur with that of an ensemble of TM-complexes in the solid-state. Finally, although the presence of methyl groups at the 3-position of the pyrazolyl groups leads to ligand–ligand repulsion, it is actually the loss of metal–ligand bonding interactions that is mainly responsible for shifts in spin-state preferences.